It is known that in DVC (Digital Video Cassette) digital video magnetic tape recording systems, a high data rate (25-50 Mb/s) and long the playing time are achievable. MPEG (Motion Picture Expert Group) has a very complex data reduction system with motion estimation, interpolation and bidirectional prediction. Great complexity arises in the encoder in which mathematical operations must be generated from the source material. To the decoder, inverse computing instructions are conveyed which reconstructs the image. This coding process is very effective but the great complexity of the encoder with little complexity in the decoder provides a system adapted to be mainly used in the area of transmission systems for digital image material, for example, digital television with data rates between 2 to 8 Mb/s for SDTV and 6 to 24 Mb/s for HDTV. In this arrangement, a complex encoder is needed in the studio for each program to be transmitted, and the incoming data stream is interpreted by simple decoders at the user.
Since it is also intended to record these signals digitally at the data-reduced level, digital magnetic tape recording devices according to the DVC system, are considered usable. In this arrangement, the MPEG data stream is recorded transparently, i.e., unchanged, and is outputted during playback so that the decoder does not see any difference between transmitted and recorded data.
Such a recording device is capable of a continuous recording data stream of, for example, 25 Mb/s, but only a continuous/variable data stream of, for example, 8 Mb/s (SDTV) is available. Accordingly, an adaptation of the recording data rate is necessary to use the full capability of the recorder and the available excess capacity of the recorder should not be lost but should be utilized for extending the playing time.
The present invention has an object of creating a recording method by means of which the existing storage capacity of a magnetic tape can be better utilized Accordingly, it is an object of the present invention to provide a recording method for DVC devices, which has an extension of playing time by providing a constant speed tape transport with part-segmented helical track recording and a bi-directional mode during recording and playback.
In known methods, there is a phase of tape rewinding between the segment writing processes, in which no recording takes place for approximately 4 minutes (depending on transport speed and tape length) unless this amount of data is accommodated in a memory. With an input data rate of 8 Mb/s and 4 minutes duration, the memory would have to have a size of 1920 Mbit. Such a memory is too expensive for consumer products.
In the method according to the present invention, this can be achieved by initially writing only a part-area of the helical track (for example 1/4), a so-called segment. The remainder of the track (for example 3/4) initially stays blank or is filled with data which is not relevant. When a number of passes follow one another, more and more areas of the track are recorded until, finally, the total track is completely recorded.
The helical track magnetic tape recording device according to aspects of the invention exhibits a bidirectional recording and replay mode, the tape transport speed being identical in both directions and in both operating modes. The principle of segmented recording is used. When the first segment has been recorded in the forward direction of transport and the end of the tape is reached, the direction of tape travel changes and the second segment is added with a changed track angle to segment 1 adjoining it. The offsets between the track segments occurring due to the difference in track angle are absorbed by having correspondingly large steps between segments. To buffer the response time of the drive during the change in direction at the ends of the tape, a memory is provided which accommodates the data for, for example, 2 seconds.
The total recording consists of individual segmented single recordings in a row which partially differ in their track angle. Between the recording of the individual segments, the direction of tape travel is switched over. During the switching processes, the data is temporarily stored. The length of the segments depends on the ratio between input data rate and recording data rate.
Segments which have been written in the same direction of tape travel can be simultaneously read during one replay pass. The tracking information is only obtained from one .about. segment. During the recording, the memory filled after one switch-over process is emptied until the next process of switching tape direction occurs. During replay, the memory is always kept full in order to be able to provide data from the memory when the direction is changed. The maximum recording data rate is slightly higher on average than the input data rate. The replay data rate corresponds to the input data rate during the recording. The replay data rate is higher than the input data rate during the recording (during the simultaneous replaying of a number of segments). The data contents of an individual segment correspond to the audio or, respectively, video information of a program. At the end of the tape, the drive is switched to the inverse direction of tape transportation and the starting and turn-off time of the recording amplifier is adapted in such a manner that a track segment adjoining the previously written segment is written at a different track angle.